import java.util.StringTokenizer;

public class P11231_39643 {

       static String readLn() {
               String newLine = System.getProperty("line.separator");
               StringBuffer buffer = new StringBuffer();
               int car = -1;
               try {
                       car = System.in.read();
                       while ((car > 0) && (car != newLine.charAt(0))) {
                               buffer.append((char) car);
                               car = System.in.read();
                       }
                       if (car == newLine.charAt(0))
                               System.in.skip(newLine.length() - 1);
               } catch (java.io.IOException e) {
                       return (null);
               }
               if ((car < 0) && (buffer.length() == 0))
                       return (null);
               return (buffer.toString()).trim();
       }

       public static void main(String[] args) {



               while (true) {
                       StringTokenizer st = new StringTokenizer(readLn());

                       int a = Integer.parseInt(st.nextToken());
                       int b = Integer.parseInt(st.nextToken());
                       int c = Integer.parseInt(st.nextToken());

                       if (a == 0 && b == 0 && c == 0) break;

                       process(a, b, c);

               }

       }

       static void process(int a, int b, int c) {

               a -= 7;
               b -= 7;

               int result = a * b / 2;

               if (result * 2 == a * b) System.out.println(result);
               else
                       System.out.println(result += c);


       }




}